54 CARBONIFEROUS CKPHALOPODA OF lU ELAND. 



1891. DisciTES PLANOTiEGATts, J. H. Foord. Cat. I'Vss. Ceph. British 



Museum, pt. 2, p. 93. 



1S93. Cffii.oNAUTiLUS i*LASOTEiiOATUS, A. Hyiilt. Carboniferous Ceplialopod?. 



Second paper. Geological Survey of 

 Teias, Fourtli Annual ltf|)ort, 1892, 

 p. 407. 



Descriptio)!. — Shell very thick-discoid, with subquadrate whorls, a deep 

 umbilicus with step-like declivities from one whorl to the other, and a small 

 central vacuity. The section of the whorls is hexagonal, and slightly concave 

 along the median line of the peripheral area; the zone of impression is rather 

 shallow. The body-chamber is rather large, and occupies nearly half of the last 

 whorl. The rate of increase in the whorls is slow, and their width is about equal 

 to that of the periphery. The umbilicus is wide and deep, and its step-like 

 character very conspicuous. The sides are angular, both at the edge of the peri- 

 pheral area and at that of the umbilicus, but there are no keels. In aged 

 individuals the angularity of the umbilical margin is greatly lessened on the 

 body-chamber in the vicinity of the aperture (Pi. XIX, fig. 1 a). The character of 

 the aperture can only be inferred from the sinuous lines on the peripheral area, 

 which show it to have had a moderately deep hyponomic sinus. The inner 

 whorls exhibit the same quadrate character as the outer ones, which they resemble 

 in miniature. Two obscure longitudinal elevations are present on the peripheral 

 area of the young shell, which leave a very shallow median depression and a 

 slight depression between them and the edge of the area. As the shell advances 

 in growth, fine longitudinal thread-like lines or ridges are developed upon the 

 peripheral area on each side of the shallow median depression, and in the 

 proximity of the margin. These appear to become obsolete in the adult shell, as 

 far as the material at my disposal enables me to judge. The surface of the test 

 is covered witli fine striee of growth. These are well figured by de Koninck 

 (' Calc. Carb.,' pt. 1, pi. xxvi, figs. 2, 3 a, 3 c). 



The septa are fairly approximate. In the young shell, where the width of the 

 peripheral area is 13 mm., they are 4 mm. apart in the median line; at a width 

 of 17 mm. this distance has increased to 7 mm., and in the adult shell, at a width 

 of 24 mm., they are 8 mm. apart, and in a large individual they are 12 mm. 

 apart where the side measures 40 mm. across. The sutures are curved deeply 

 backwards on the sides of the shell, and form sharply bent-back lobes upon the 

 peripheral area. The sutural characters in this species are thus clearly marked, 

 and are important aids in its identification (PI. XIX, figs. Ih, 2 a, 2 b). 



The siphuncle, as seen in a natural section of a septum, is situated in the 

 upper third, that is considerably above the centre (PI. XIX, fig. 2 c). 



Dimensions. — M'Coy affirms that this species often attains a diameter of lU 



